The invention relates to a monolithic or integrated structure for detection or infrared imaging, as well as to its production process. It makes it possible to produce industrially on a large scale pyroelectric infrared cameras, whose complexity and spacing of the elementary sensors are directly compatible with a display on an existing standard television screen.
At present, the development of civil applications of infrared imaging, such as medical applications and the monitoring of areas with a high fire risk is technically limited by the lack of an industrial technology concerning inexpensive, general public detectors.
Infrared detection means are based on the property of a certain number of dielectric materials of having a spontaneous polarization, i.e. a residual internal electric field, which varies as a function of the temperature. This property is the pyroelectric effect.
The temperature variation is proportional to the energy of the incident photon signal. For a given energy, it increases with the decrease in the thermal capacity of the detector, which leads to the use of a thin pyroelectric layer for minimizing the detection volume.
The polarization variation is manifested at the detector terminals by a charge variation. Two metal electrodes deposited on the surface of the dielectric make it possible to constitute a capacitor, at whose terminals it is possible to measure the voltage when it is read by a high impedance input circuit of the MOS transistor type. This voltage of a few millivolts is proportional to the incident energy variation.
The generally used pyroelectric materials are given in the article "Pyroelectric Devices and Materials" by R. W. Watermoore, Rep. Prog. Phys. 49, 1986, pp 1335-1386. Reference is made among these materials to polymers or copolymers of the polyvinylidene fluoride type.
FIG. 1 diagrammatically shows in perspective, a first embodiment of a known infrared detector, more particularly described in "Ferroelectrics for infrared detection and imaging" by R. Watton, IEEE Int. Symp. on Applications of Ferroelectrics, June 1986, Bethlehem, U.S.A. In this detector, the detecting part is connected to the reading circuit by a hybridization process.
As shown in FIG. 1, each sensor 2 of the detector has two electrodes 4 and 6 placed on opposite faces of the pyroelectric material 8. The polarization vector 9 must be perpendicular to the structure of the electrodes 4 and 6. A thermal insulating material 12 insulates each sensor 2 from a soldered joint on metal stud 14 for hybridization. For each sensor, a metallization 16 then ensures the contact between the electrode 6 and the soldered joint 14.
The sensors 2 constituting the elementary points in imaging are thermally and mechanically insulated or isolated from one another by the etching or pyroelectric materials 8, conductor 6 and insulant 12. This insulation or isolation carries the reference 15.
The reading of the voltage on electrode 6 resulting from the interaction of an infrared radiation 17 with the detector takes place via control lines 22, in an integrated circuit 18 made from silicon, with the aid of a single input amplifier stage 20, the electrode 4 common to all the sensors 2 serving as a reference for all these sensors. To simplify the description and drawings, the integrated circuit is only shown in its functional form.
This arrangement of the detector prevents a gain exceeding 100. Moreover, this technology requires a large number of masks and in particular six masking levels for producing the detecting part.
A more recent construction of a detection device is diagrammatically shown in perspective in FIG. 2. This is more particularly described in the article "Type II pyroelectric detectors" by A. Hadni, Infrared Physics, vol. 27, No. 1, pp 17-23, 1987.
In this construction, the two electrodes 4a and 6a of each sensor 2a are placed on the same face of the pyroelectric material 8a and are respectively connected to the reading circuit 18a by two soldered joints 14a, 14b. In this structure, the polarization vector 9a must be parallel to the plane of the electrodes 4a, 6a. The detection-useful zone, indicated by mixed lines and designated 24, is approximately the volume having as its base the surface between the electrodes 4a, 6a and whose height is equal to the distance d between these electrodes.
This structure is preferable to that shown in FIG. 1, because there is no common electrode, which is not well supported in the insulation zones 15 between each sensor. The common electrode is consequently fragile and unreliable. Moreover, the two electrodes 4a, 6a are located on the hydridization face and make it possible for each sensor 2a to be read by a differential amplifier 20a.
It is known that this type of amplifier eliminates the noise occurring in the common mode, such as the piezoelectric noise and thus, for each sensor, allows an amplification with a gain which can reach 1000.
In this embodiment, metallizations 16a and 16b respectively ensure the electrical contact between electrodes 4a, 6a and soldered joints 14a, 14b. Moreover, a thermal insulating material 12a insulates each sensor 2a from its two soldered joints. Once again, the thermal and mechanical insulation 15a of the sensors is ensured by etching the insulating layer 12a and the pyroelectric layer 8a.
The two aforementioned embodiments suffer from a certain number of disadvantages.
Thus, the metal connections 16, 16a, 16b between the sensor and the integrated circuit constitute thermal points limiting the temperature rise of the detector and therefore reducing by the same amount the voltage variation, whereas the reading noise is not reduced. Thus, the detection of the infrared signal is greatly deteriorated.
Moreover, the topological constraints linked with the construction of the soldered joints do not make it possible to envisage a sensor size smaller than 40 .mu.m. This size exceeds the size of the diffraction spot of the optics associated with the detector, so that the sensors limit the angular shooting resolution.
Moreover, the hybridization operation is not a standard technological step in microelectronics. Moreover, the methods usable for the deposition and etching of electrodes do not make it possible to move them together to a distance d (FIG. 2), which is less than about 10 micrometers. Therefore the capacity of the sensor is low compared with the input capacity of the reading circuit. Therefore the pyroelectric voltage supplied by the detector is reduced.
In addition, the cavity 26 beneath each sensor 2a of FIG. 2 contains traces of the products used during the production process. These products remain active when the detection circuit is finished, which leads to a reduction in the life of said device.
These disadvantages apply no matter which pyroelectric material is used. However, they are much more marked when using as the sensitive material a polymer or copolymer of the polyvinylidene fluoride type. In particular, the hybridization operation excessively raises the temperature of the polymer and reduces its performance characteristics.
Moreover, the metal deposit necessary for producing the electrodes on a very thin polymer film induces mechanical constraints therein, which deform the film and the metal layer. The thus deformed metal layer becomes unsuitable for the production of etched electrodes using microelectronic methods.
In conclusion, the hybridization method of sensors with an integrated circuit does not make it possible to produce detectors or infrared imaging means, which are reproducible and reliable and have a high integration density. The integration of the reading circuits and sensors is dealt with in a publication entitled "Fully-integrated ZnO on silicon pyroelectric infrared detector array" by D. L. Polla et al, Proceeding IEDM, 1984, pp 282-284.
The technology described in this article is not suitable for high density integration. Thus, the reading circuit and the sensor connected thereto are not located in superimposed manner as described relative to FIGS. 1 and 2. Moreover, the thickness reduction of the silicon necessary for the detection cannot be obtained for a small spacing less than 10 micrometers.